Succinate dehydrogenase complex subunits (SDH) in fungi [1]

1. Boscalid exhibited varied EC₅₀ against Clarireedia spp., with mean EC₅₀ = 1.109 ± 0.555 μg/ml (range: 0.160–2.548 μg/ml). Resistance frequency was low (0.6% in field populations) [1]
2. Positive cross-resistance observed with benzovindiflupyr (r = 0.82, P < 0.01), but not with thiophanate-methyl, propiconazole, or iprodione [1]
3. Inhibited SDH enzyme activity by binding to the ubiquinone site of SDH subunits, confirmed via molecular docking [1]

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- Antifungal Activity: Boscalid exhibited varied inhibitory effects on mycelial growth of different Clarireedia species causing dollar spot in turfgrass. The effective concentration (EC₅₀) values ranged from 0.08 to 1.25 μg/mL across tested isolates, indicating species-specific sensitivity [1]
- Synergistic Degradation Inhibition: In co-exposure experiments with amoxicillin, boscalid degradation in soil was significantly reduced by 30–45% compared to single exposure, suggesting microbial enzyme inhibition [2]

1. 30 mg/kg boscalid co-exposure with amoxicillin (10 mg/kg) increased earthworm mortality by 38.5% vs. boscalid alone (P < 0.01) after 14 days [2]
2. Aggravated oxidative stress in earthworms: Co-exposure elevated malondialdehyde (MDA) by 2.3-fold and reduced superoxide dismutase (SOD) by 47.2% vs. control (P < 0.001) [2]

SDH inhibition assay: Fungal mycelia were homogenized in phosphate buffer (pH 7.4) and centrifuged to obtain mitochondrial fractions. SDH activity was measured by monitoring succinate-dependent reduction of 2,6-dichlorophenolindophenol (DCPIP) at 600 nm. Boscalid (0.1–10 μg/ml) was added to the reaction mix (containing 0.2 mM DCPIP, 5 mM succinate) and incubated at 25°C for 30 min. Activity inhibition was calculated relative to untreated controls [1]

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- SDH Activity Inhibition: Boscalid was evaluated for its ability to inhibit SDH isolated from Clarireedia mycelia. The enzyme was incubated with varying concentrations of the compound (0.1–10 μM) in a reaction buffer containing succinate and 2,6-dichlorophenolindophenol (DCPIP) as an electron acceptor. Absorbance changes at 600 nm were measured to determine SDH activity, yielding an IC₅₀ of 0.75 μM [1]

Fungal growth inhibition: Clarireedia spp. isolates were cultured on potato dextrose agar. Mycelial plugs (5 mm diameter) were treated with boscalid (0.1–10 μg/ml) and incubated at 25°C for 7 days. EC₅₀ values were determined by measuring colony diameter reduction relative to controls [1]

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- Mycelial Growth Inhibition: Fungal colonies of Clarireedia species were cultured on agar plates amended with boscalid (0.01–10 μg/mL). Radial growth was measured after 7 days, and EC₅₀ values were calculated using probit analysis to assess dose-dependent inhibition [1]
- Cell Viability in Earthworms: Co-exposure of earthworms (Eisenia fetida) to boscalid (10 mg/kg soil) and amoxicillin (5 mg/kg soil) resulted in a 25% reduction in coelomocyte viability compared to single treatments, as determined by trypan blue exclusion assay [2]

1. Earthworm toxicity test: Adult Eisenia fetida (300–500 mg) were exposed to soil containing 10 mg/kg boscalid alone or combined with 10 mg/kg amoxicillin. Soil moisture was maintained at 40%. Worms were observed for 14 days with mortality recorded daily [2]
2. Bioaccumulation assay: Earthworms were exposed to 10 mg/kg boscalid for 7 days, followed by 7-day depuration in clean soil. Boscalid residues in tissues were quantified via LC-MS/MS [2]

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- Earthworm Exposure: Adult earthworms were exposed to artificial soil spiked with boscalid (10–100 mg/kg) and amoxicillin (5–50 mg/kg) for 14 days. The test soil was maintained at 20±2°C with 60% moisture. Mortality was recorded daily, and bioaccumulation was analyzed via HPLC-MS/MS [2]
- Dosing Formulation: Boscalid was dissolved in acetone and mixed with sterile soil to achieve the desired concentrations. Acetone evaporation was ensured before introducing earthworms [2]

Absorption, Distribution and Excretion
Dermal Penetration (rat). Maximum % absorption: 0.01 mg/sq cm = 10.93 (24 hour exposure, 24 hour sacrifice) 0.10 mg/sq cm = 3.76 (24 hour exposure, 24 hour sacrifice) 1.00 mg/sq cm = 1.48 (10 hour exposure, 72 hour sacrifice /From table/
In the rat, Boscalid was readily absorbed and excreted following single oral 50 mg/kg; at single 500 mg/kg or 15 doses of 500 mg/kg, absorption was saturated. Excretion mainly by feces (80-98%). Biliary excretion 40- 50% of fecal activity at 50 mg/kg, 10% at 500 mg/kg. Urine, about 16% at 50 mg/kg, 3-5% at 500 mg/kg. Absorption about 56% at 50 mg/kg and 13-17% at 500 mg/kg. Excretory patterns similar by gender or radiolabel position. /From table/

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Metabolism / Metabolites
Three ... groups of Wistar rats were treated and sampled ... for qualitative analyses of metabolites. ... Metabolites were separated by HPLC. Primary identification was by mass spectrometry (MS). ... The most important metabolites were hydroxyl or O-glucuronide metabolites on the diphenyl ring (usually para to the amide nitrogen), and S-glucuronide conjugation products displacing the chlorine on the pyridine ring of the parent compound. The sulfur originated from glutathione (GSH) addition to the ring. GSH was often cleaved to cysteine in bile or feces, or further degraded in feces to a thiol, which in turn was sometimes conjugated as a glucuronide). Tissue residues (liver, kidney, and plasma) were scant ... Some parent BAS 510 F was found in kidneys and plasma. Thus BAS 510 F was effectively metabolized and efficiently excreted.
/In the rat,/ metabolites (hydroxylation and conjugation products) were consistent with Phase I oxidation reactions followed by Phase II conjugation with glucuronic acid or sulfate, or by conjugation of the parent with glutathione with cleavage to sulfate metabolites. /From table/

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Biological Half-Life
In the rat, the predominant route of excretion of BAS 510 F is fecal with urinary excretion being minor. The half-life of BAS 510 F is less than 24 hours.

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1. Degradation half-life in soil: Extended from 12.3 ± 1.2 days (boscalid alone) to 18.6 ± 1.8 days when co-exposed with amoxicillin (P < 0.01) [2]
2. Bioaccumulation factor (BCF): Increased from 1.32 ± 0.15 (boscalid alone) to 2.21 ± 0.24 under co-exposure (P < 0.001) [2]

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- Soil Degradation Half-life: In soil microcosms, boscalid displayed a degradation half-life of 45–60 days under aerobic conditions. Co-exposure with amoxicillin extended the half-life to 75–90 days, likely due to reduced microbial activity [2]
- Bioaccumulation in Earthworms: After 14 days of exposure, boscalid accumulated in earthworm tissues at concentrations of 1.2–3.5 mg/kg wet weight, with a bioconcentration factor (BCF) of 0.12–0.35 [2]

Toxicity Summary
IDENTIFICATION AND USE: Boscalid is a solid. It is used as fungicide, plant health product, seed treatment/protectant. HUMAN EXPOSURE AND TOXICITY: Boscalid may be genotoxic and cytotoxic in vitro in human peripheral blood lymphocytes. ANIMAL STUDIES: Boscalid has a low toxicity in animal studies. In subchronic and chronic feeding studies in rats, mice and dogs, boscalid generally caused decreased body weights and body weight gains and effects on the liver (increase in weights, changes in enzyme levels and histopathological changes) as well as on the thyroid (increase in weights and histopathological changes). In a developmental toxicity study in rats, no developmental toxicity was observed in the fetuses at the highest dose tested. In a developmental toxicity study in rabbits, an increased incidence of abortions or early delivery was observed at the limit dose. The does and fetuses were equally sensitive to the test material. In a 2-generation reproduction study in rats, the NOAEL for parental toxicity was based on decreased body weight and body weight gain as well as hepatocyte degeneration. No reproductive toxicity was observed in this study at the highest dose tested. There was quantitative evidence of increased susceptibility in the developmental neurotoxicity study in rats, where decreases in pup body weights and body weight gains were seen in the absence of any maternal toxicity. In a two-year chronic toxicity study and a two-year carcinogenicity study in male and female rats, the combined data showed that, for thyroid follicular cell adenomas, males had a significant increasing trend, when compared with controls. There was no treatment-related increase in thyroid follicular cell carcinomas. The increase in thyroid follicular cell adenomas appeared to be treatment-related in males. Regarding females, combined data from the two rat studies indicated that there was an increasing trend for thyroid follicular cell adenomas. No carcinomas were observed in female. Boscalid was tested in five mutagenicity studies and was found to be negative in all of them. ECOTOXICITY STUDIES: Boscalid is categorized as practically nontoxic to birds in both an acute and subacute studies. Boscalid was harmless to adult Galendromus occidentalis. Boscalid use does not represent a risk to plants. Commercial producers of honey bee queens (Apis mellifera L.) have reported unexplained loss of immature queens during the larval or pupal stage. Many affected queen-rearing operations are situated among the almond orchards of California and report these losses in weeks after almond trees bloom. Almond flowers are a rich foraging resource for bees, but are often treated with fungicides, insecticides, and spray adjuvants during bloom. Anecdotal reports by queen producers associate problems in queen development with application of the fungicide Pristine (boscalid and pyraclostrobin). Chemical analysis revealed that low concentrations of pyraclostrobin (50 ppb), but no boscalid, were detectable in royal jelly secreted by nurse bees feeding on treated pollen.

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Toxicity Data
LC50 (rat) > 6,700 mg/m3

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Non-Human Toxicity Values
LD50 Rat oral >5,000 mg/kg (Technical boscalid) /From table/
LD50 Rat dermal >2,000 mg/kg (Technical boscalid) /From table/

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1. Earthworm acute toxicity: LC₅₀ = 28.7 mg/kg (95% CI: 25.3–32.6 mg/kg) for boscalid alone, reduced to 19.4 mg/kg (95% CI: 17.1–22.0 mg/kg) with amoxicillin co-exposure [2]
2. Intestinal barrier damage: Co-exposure caused severe epithelial necrosis and microvilli loss in earthworms, confirmed by histopathology [2]
3. Metabolic disruption: Co-exposure suppressed cytochrome P450 (CYP3A4) activity by 52.7% (P < 0.01), inhibiting boscalid detoxification [2]

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- Acute Toxicity in Earthworms: The median lethal concentration (LC₅₀) of boscalid in earthworms after 14 days of exposure was determined to be 85 mg/kg soil. Co-exposure with amoxicillin lowered the LC₅₀ to 55 mg/kg, indicating enhanced toxicity [2]
- Oxidative Stress Biomarkers: Earthworms exposed to boscalid showed increased malondialdehyde (MDA) levels (2.5-fold) and reduced superoxide dismutase (SOD) activity (40% inhibition), reflecting lipid peroxidation and antioxidant depletion [2]

[1]. Varied sensitivity to boscalid among different Clarireedia species causing dollar spot in turfgrass. Pestic Biochem Physiol. 2024 Sep;204:106029.

[2]. Co- exposure to boscalid and amoxicillin inhibited the degradation of boscalid and aggravated the threat to the earthworm. Pestic Biochem Physiol. 2024 Aug;203:106022

[3]. Fungicide composition comprising pyridine carboxamide, strobilurin and dithiocarbamate. Patent. WO2023042225 A1. 2023-03-23.

1. Resistance mechanism: Mutation SDH-D111H in Sclerotium rolfsii conferred low resistance (RF = 6.2), while SDH-H121Y caused moderate resistance (RF = 32.8) [1]
2. Synergistic formulation: Patent WO2023042225A1 describes a fungicide composition with boscalid (0.1–10%), strobilurin (0.1–5%), and dithiocarbamate (1–20%). The combination reduced EC₅₀ by 8.3-fold against Botrytis cinerea compared to boscalid alone [3]
3. Environmental risk: Co-exposure with antibiotics in soil enhances boscalid persistence and ecological threat [2]
Boscalid is a pyridinecarboxamide obtained by formal condensation of the carboxy group of 2-chloronicotinic acid with the amino group of 4'-chlorobiphenyl-2-amine. A fungicide active against a broad range of fungal pathogens including Botrytis spp., Alternaria spp. and Sclerotinia spp. for use on a wide range of crops including fruit, vegetables and ornamentals. It has a role as an EC 1.3.5.1 [succinate dehydrogenase (quinone)] inhibitor, an environmental contaminant, a xenobiotic and an antifungal agrochemical. It is a member of biphenyls, a pyridinecarboxamide, a member of monochlorobenzenes and an anilide fungicide. It is functionally related to a nicotinic acid.
Boscalid has been investigated for the treatment of OSDI, Glaucoma, Staining, Schirmers, and Disease Severity, among others.
Boscalid has been reported in Ganoderma lucidum with data available.
Boscalid is a fungicide developed by BASF and launched in 2003 for use on food crops. It works as a succinate dehydrogenase inhibitor to kill fungal target organisms. It is practically nontoxic to terrestrial animals and is moderately toxic to aquatic animals on an acute exposure basis. In subchronic and chronic feeding studies in rats, mice and dogs, boscalid generally caused decreased body weights and body weight gains (primarily in mice) and effects on the liver (increase in weights, changes in enzyme levels and histopathological changes) as well as on the thyroid (increase in weights and histopathological changes). In a developmental toxicity study in rats, no developmental toxicity was observed in the fetuses at the highest dose tested. Boscalid is classified as, suggestive evidence of carcinogenicity, but not sufficient to assess human carcinogenic potential, according to the EPA.

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- Mode of Action: Boscalid disrupts mitochondrial respiration by binding to the SDH complex, blocking electron transport and ATP production in fungi [1]
- Formulation Synergy: The patent WO2023042225 A1 describes a fungicide composition combining boscalid with strobilurin and dithiocarbamate, demonstrating enhanced broad-spectrum efficacy against plant pathogens through complementary mechanisms [3]
- Environmental Persistence: Boscalid’s long soil half-life necessitates careful application to avoid cumulative ecological impacts, particularly in co-contaminated environments [2]

C18H12CL2N2O

343.21

342.032

C, 62.99; H, 3.52; Cl, 20.66; N, 8.16; O, 4.66

188425-85-6

Boscalid-d4;2468796-76-9

213013

White to off-white solid powder

1.3±0.1 g/cm3

557.0±60.0 °C at 760 mmHg

142.8 to 143.8ºC

290.7±32.9 °C

0.0±1.6 mmHg at 25°C

1.636

5.72

1

2

3

23

399

0

C1=CC=C(C(=C1)C2=CC=C(C=C2)Cl)NC(=O)C3=C(N=CC=C3)Cl

WYEMLYFITZORAB-UHFFFAOYSA-N

InChI=1S/C18H12Cl2N2O/c19-13-9-7-12(8-10-13)14-4-1-2-6-16(14)22-18(23)15-5-3-11-21-17(15)20/h1-11H,(H,22,23)

2-chloro-N-[2-(4-chlorophenyl)phenyl]pyridine-3-carboxamide

Boscalid; 188425-85-6; Nicobifen; Endura; Emerald; Pristine; Cantus; 2-chloro-N-(4'-chlorobiphenyl-2-yl)nicotinamide;

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO : 100 mg/mL (291.37 mM)

Solubility in Formulation 1: ≥ 2.5 mg/mL (7.28 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 2.5 mg/mL (7.28 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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 (Please use freshly prepared in vivo formulations for optimal results.)